Start with loud and clear pronunciation, where difference is obvious. Practice and you will end with fluent speech, where unstressed vowels are degraded to some kind of "unclear sounds". It is biology and physics of mouth and language.

Loud and clear pronounciation makes sense to get a better feeling for it. I might record myself a couple of times and post it here. Maybe someone can then tell me how far off my pronounciation still is

осенняя as-sen-n'uh-juh
я
This is how i imagine saying it. -ие -ее the same, whereas the -ии wouldn't have the Y-sound for the last vowel. And the -яя would be the schwa sound of fathEr, with the last vowel a Y-sound to it.